In the area of bathroom tissue, softness, absorbency and strength are key attributes considered by consumers. It is highly desirable that the tissue product have a consumer perceived feel of softness. This softness plays a key role in consumer preference. Softness relates both to the product bulk and surface characteristics. In addition to softness, the consumer desires a product that is both strong and absorbent to minimize the amount of the product which must be used to do an effective job.
In a conventional wet press (“CWP”) process and apparatus 10, as exemplified in FIG. 1, a furnish is fed from a silo 50 through conduits 40, 41 to headbox chambers 20, 20′. A web (W) is formed on a conventional wire former 12, supported by rolls 18, 19 from a liquid slurry of pulp, water and other chemicals. Materials removed from the web of fabric in the forming zone when pressed against a forming roll 15 are returned to the silo 50 from a saveall 22 through a conduit 24. The web is then transferred to a moving felt or fabric 14, supported by a roll 11 for drying and pressing. Materials removed from the web during drying and pressing or from a Uhle box 29 are collected in a saveall 44 and fed to a white water conduit 45. The web is then pressed by a suction press roll 16 against the surface of a rotating Yankee dryer cylinder 26 which is heated to cause the paper to substantially dry on the cylinder surface. The moisture within the web as it is laid on the Yankee surface causes the web to transfer to the surface. Liquid adhesive may be applied to the surface of the dryer to provide substantial adherence of the web to the creping surface. The web is then creped from the surface with a creping blade 27. The creped web is then usually passed between calender rollers 30 and rolled up on a roll 28 prior to further converting operations, for example, embossing. The action of the creping blade on the paper is known to cause a portion of the interfiber bonds within the paper to be broken up by the mechanical smashing action of the blade against the web as it is being driven into the blade. However, fairly strong interfiber bonds are formed between the wood pulp fibers during the drying of the moisture from the web. The strength of these bonds in prior art tissues is such that, even after creping, the web typically retains a perceived feeling of hardness, a fairly high density, and low-bulk and water absorbency.
To reduce the strength of the interfiber bonds that inevitably result when wet pressing and drying a web from a slurry, various processes have been utilized. One such process is the passing of heated air through the wet fibrous web after it is formed on a wire and transferred to a permeable carrier—a so-called through-air-drying (“TAD”) process—so that the web is not compacted prior to being dried. The lack of compaction, such as would occur when the web is pressed while on a felt or fabric and against the drying cylinder when it is transferred thereto, reduces the opportunity for interfiber bonding to occur, and allows the finished product to have greater bulk than can be achieved in a wet press process. Because of the consumer perceived softness of these products, and their greater ability to absorb liquids than webs formed in wet press processes, the products formed by the newer TAD processes enjoy an advantage in consumer acceptance.
Felted wet press processes, however, are significantly more energy efficient than processes such as through-air-drying since they do not require heating and moving large quantities of air as required by the TAD process. In wet press operations, excess moisture is mechanically pressed from the web and the final drying of the web is obtained chiefly on the heated Yankee drying cylinder which is maintained at the proper drying temperature.
A conventional TAD process is illustrated in FIG. 2. In the process, a wet sheet 71 that has been formed on a forming fabric 61 is transferred to a through-air-drying fabric 62, usually by a vacuum device 63. TAD fabric 62 is usually a coarsely woven fabric that allows relatively free passage of air through both the fabric 62 and nascent web 71. While on the fabric 62, the sheet 71 is dried by blowing hot air through the sheet 71 using a through-air-dryer 64. This operation reduces the sheet moisture to a value usually between 10 and 65 percent. The partially dried sheet 71 is then transferred to a Yankee dryer 30 where it is dried to its final desired moisture content and is subsequently creped off the Yankee.
In a conventional embossing process according to the prior art as shown in FIG. 3, cellulosic webs 11 and 12 are supplied from parent rolls 21 and 22. The webs are joined together at or prior to the emboss nip which is formed by an emboss roll 31 and a backing roll 41. The emboss roll 31 has raised protrusions (not shown) on the surface of the roll that make up the emboss pattern. This pattern is transferred to the combined webs by pressing them between the emboss roll 31 and the backing roll 41, which is usually made of rubber or other like material that allows the pattern on the emboss roll 31 to be impressed into the combined web 13. The combined web 13 may be further processed by perfing and/or calendering (not shown) before it is wound onto a finished roll 51.
Thus, the art lacks a method for making a CWP tissue product that achieves high strength, bulk, absorbency, and softness above existing conventional wet pressed tissue, approaching or achieving levels even beyond those found using through-air-drying. The art further lacks a method for making a TAD product that achieves high strength, bulk, absorbency, and softness above existing TAD tissue.
These and other drawbacks of the prior art are overcome by the method of the present invention which can use wet press technology to prepare a strong, ultra soft, bulky tissue. The tissue produced by the method of the present invention exhibits good strength and absorbency while remaining extremely soft. Properties such as those exhibited by the CWP tissue of the present invention have not heretofore been seen in wet press tissue products. The present invention provides a method for making a CWP tissue product that achieves high strength, bulk, absorbency, and softness above existing conventional wet pressed tissue, approaching or achieving levels even beyond those found using through-air-drying. The process according to the present invention is capable of using the cheaper more efficient wet press process and optionally can use less expensive, non-premium fibers. The method of the present invention can also be used to prepare multi-ply tissue products from TAD basesheets. The present invention can be used to make a TAD product that achieves high strength, bulk, absorbency, and softness above existing TAD tissue.